Historically, the geometry of knife blades has varied significantly in length, thickness and shape of the blade. This has been particularly the case for sport, pocket knife and tactical knife blades.
In order to control the sharpening angle, the face of the knife blade is laid on a planar surface, acting as the angle guide, and held by hand, a magnet or a spring against that surface.
Because of the geometric complexity and variety of thickness of the sports and tactical knife blades, the positioning of these type of blades on such planar angle guides is unstable and ambiguous. Consequently, the precise angular control of the edge facets to be sharpened are further compromised when the knife blades are excessively thick and the blade is held in place against the guide by a spring which creates increasing friction as the blade is pulled through the sharpening slot.